The present invention relates to a mobile communications system.
In a prior art mobile communications system, a base station (connecting unit) is connected to a switched telephone network and radio speech channels are established between it and portable telephones. A control channel is used during channel acquisition and release to transmit control signals including a control signal. The base station constantly monitors the status of calls and the request of a call from mobile units by sequentially scanning the speech channels and control channel. The control signal of the system includes a preamble which is a 12-bit sequence of alternating 1's and 0's for clock timing recovery, a frame sync field containing a 16-bit sequence of "1100010011010110", and a 37-bit calling sequence which is a 25-bit user's identification code encoded with 12 error correcting bits. The base station recovers clock timing from the preamble of a control signal received on the control channel and proceeds to detect a frame sync code from the subsequent frame sync field of the control signal. Upon detection of a frame sync, the base station stores the following calling sequence into a buffer by counting the recovered clock pulses. However, the error correction encoding can result in a bit sequence identical to the 16-bit frame sync code. For example, encoding of an identification code "806240" results in a 37-bit code "0000011000100110101100000000001011011" which contains a sequence having the same bit pattern as the 16-bit frame sync code as identified by an underline. To prevent the stored calling sequence from being disrupted by a false sync code, the base station is "locked" into a state in which it is insensitive to the subsequent frame sync until an entire 37-bit calling sequence is stored into the buffer.
One shortcoming of this approach is that, due to the constant scanning operation, the base station may switch from one speech channel to the control channel at the instant a calling sequence is being received. If this calling sequence contains a false sync code, and if clock timing is shortly recovered, such false sync code will be detected and the buffer will be supplied with information other than the intended calling sequence. The result is a decision by a decoder that a mismatch has occurred between the received user's identification code and a stored version of the code. Since the base station remains locked in response to a frame sync until the buffer is completely supplied with a 37-bit sequence, the true sync code of the next control signal can occur in this locked state, and the base station will be locked again following the next false sync code, and a mismatch decision is repeated. This process continues indefinitely.